Interception and analysis of communications and radar signals plays a significant role in a number of applications including, but not limited to, analysis of signal bandwidths for spectral congestion analysis, signal characterization for regulatory enforcement, analysis of signal characteristics for intelligence gathering and possibly jamming of the intercepted signals in order to disrupt enemy communications. Signals of interest include transmissions wherein the signal frequency either is constant or varies with time (such as frequency-hopping radios or frequency-agile radars) and wherein the signal duration is either constant or varies with time. Reasons for using frequency-hopping or agile transmissions include making the signal more difficult to intercept and/or jam (low probability of intercept or LPI), and making signal reception more robust for the intended receiver by introducing diversity or redundancy over multiple frequencies.
The intended receiver of the signal transmissions has adequate knowledge of the key transmission characteristics (such as the hopping frequencies of the frequency-hopping radio) to extract the information-bearing signal of interest and the signal information thereafter. On the other hand, the interceptor generally has no access to such knowledge. Indeed, the interceptor generally lacks knowledge of even the existence of signals in the bandwidth of interest at any given time, or of how many signal emitters there are. Clearly the interceptor must extract any required information by monitoring and processing the bandwidth of interest.
This total lack of knowledge about the characteristics of the transmissions means that the interceptor is at a disadvantage with respect to noise relative to the intended receiver, assuming that the intended receiver is properly designed. It is therefore important to minimize this penalty in order to maximize the effectiveness of the intercept receiver. This is accomplished by careful design of the intercept receiver.
The presence of multiple signal emitters creates additional problems for an intercept receiver. They can interfere with the detection of each other, as well as force the interceptor to duplicate processes (filtering, etc.) required in the estimation of their parameters. In addition, the sequence of hops that belong to each emitter needs to be resolved.
The fine-grained signal information potentially obtainable with an intercept receiver is considerable (see “A Fast Software Implementation of a Digital Filter Bank Processor for Analyzing Frequency Hopping Signals,” Al Premji, Tim J. Nohara, Robert Inkol and William Read, presented at the TTCP Digital Receiver Technology Workshop, Ottawa, Canada, Sep. 10–12, 2001). It can extract the individual hop signals, calculate numerous estimates from these hop signals (e.g. start time, end time, hop duration, rise time, fall time, bandwidth, modulation type, bit rate, bearing, power, etc.) and then, assuming that there are multiple emitters present in the sub-band, the hops can be deinterleaved (using multi-dimensional clustering or association algorithms that operate on the estimated parameters) so that the hop signal sequence from each emitter is determined. Each emitter's hop signal sequence would then form part of a message that could, in theory, be demodulated and decoded (i.e. signal exploitation) to the same degree that single-channel signals can be exploited.
In order to support jamming of frequency-hopping signals (or other LPI signals), intercept receivers must be able to disrupt the majority of the signal information contained in each signal hop. This means that the intercept receiver must be able to detect and locate (in frequency, and possibly bearing) a given hop signal within a fraction of the hop signal's duration. For example, if a 10 millisecond (ms) hop signal (i.e. hop rate of the radio is approximately 100 hops/sec) is assumed, the hop signal would ideally be detected and located in about 1 ms, so that a jamming signal can be transmitted to disrupt signal reception by the intended receiver for the remainder of the signal duration.
The fine-grained signal information obtainable is considerable if an intercept receiver dwells long enough within a sub-band occupied by a frequency hopper. For the 10 ms hop signal example, it could dwell in each sub-band for say 100 ms, detect the individual hops (about ten of them for each emitter), extract the individual hop signals, calculate estimates from these hop signals, and then deinterleave. Such fine-grained signal information is more easily obtainable using digital processors. As a result, even if the front-end receiver is analog, once signals are extracted for fine-grained analyses, the remaining processing, such as parameter estimation and deinterleaving (deinterleaving is the process of determining the number of emitters present and creating hop sequences for each emitter), would normally be implemented in software.
Two technologies are possible for receiver implementation, analog processing and digital processing. Although analog processors have been the traditional choice for intercept receivers due to the computational burden of digital processing, analog processors have numerous limitations that constrain their utility. Some of the problems include: greater size, power requirements and cost, lower reliability and repeatability of components, limitations and lack of accuracy in providing important characteristics of the signals of interest, and inflexibility (in terms of changing or improving the systems or processing algorithms employed, or migrating them to other bands of operations or applications). Some of these problems are compounded by multiple signals in the bandwidth being monitored. For these reasons, emphasis has recently shifted to an increased consideration of digital techniques. A thorough treatment of analog and digital wideband receiver systems is provided in the book “Digital Techniques for Wideband Receivers,” James Tsui, Second Edition, Artech House, 2001.
For digital processing, the signal must first be converted to discrete samples that represent the signal voltage or amplitude over time. The resulting sampled signal places a considerable computational burden on a digital receiver required to process said captured signal. Indeed, this is one of the primary reasons that, at the present time, most intercept receivers are still implemented in analog form (to circumvent the requirements of digital processing). The few digital receivers that either have been implemented or have been described in concept in the open literature resort to substantially sub-optimal solutions. These sub-optimal solutions usually perform detection using a noise/interference bandwidth that far exceeds the desired signal bandwidth, thereby degrading sensitivity; or they throw away signal information (i.e. the underlying, complex narrowband hop signal is not retained for further analysis and exploitation). They must do so in order to reduce the digital processing requirements and to make the solution practical for implementation using available digital technology. As a result, the utility of these receivers for extracting valuable signal information is also limited.